In digital communication systems (e.g., high-speed optical communication systems) information sequences and control information are commonly transmitted in the form of repetitive structures referred to as “frames”. Such systems require synchronization between a transmitter and a receiver in order to recognize the presence and alignment of the frames at the receiver before any further decoding can take place. The transmitter inserts a frame alignment sequence (FAS), typically at the beginning of a frame, to determine the position of the frame in the received digitized stream. In the receiver, a frame synchronization module, referred to hereafter as “framer”, detects the FAS and monitors frame alignment once initial frame acquisition has been accomplished.
Typically, optical communication systems use an ON/OFF Keying (OOK) modulation format, and framers for such systems are known in the art. In the field of high-speed optical communication, Differential Phase Shift Keying (DPSK) and Duobinary Signaling (DBS) modulation formats can offer significant advantages over the OOK format. To provide frame synchronization and determine the intended polarity of the transmitted stream, these modulation formats require specialized frame synchronization algorithms. However, in a communication network, multiple modulation formats may be used by component digital communication systems, and it is highly desirable to have the algorithmic behavior of the framers to be independent from and cross-compatible with multiple modulation formats.
Therefore, there is a need in the art for an improved method and apparatus for frame synchronization in digital communication systems that supports multiple modulation formats.